The Montreal Protocol for protection of the ozone layer has led to discontinued use of chlorofluorocarbons (CFCs). Compounds less harsh on the ozone layer have therefore replaced chlorofluorocarbons, such as hydrofluorocarbons (HFCs). However, these compounds make a relatively large contribution towards the greenhouse effect. Efficient replacement compounds have therefore been researched having both a low ODP (Ozone Depletion Potential) coefficient and low GWP (Global Warming Potential) coefficient. Hydrofluoroolefins (HFOs) have been identified as desirable alternatives on account of their low ODP and GWP values.
Hydrofluorocarbons (HFCs) and in particular hydrofluoroolefins (HFOs), such as 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) are compounds with known properties as refrigerants and heat-transfer fluids, as fire-extinguishing and propellant agents, foaming agents, swelling agents, dielectric gases, polymerisation or monomer medium, as carrier fluids, agents for abrasives, drying agents and fluids for energy production units. Unlike CFCs and HCFCs, that are potentially dangerous for the ozone layer, HFOs do not contain chlorine and therefore do not give rise to problems for the ozone layer when used.
It is known to produce hydrofluoroolefins or hydrofluorocarbons via fluorination of hydrochloroolefins or hydrochlorocarbons in particular. This fluorination is generally a catalytic fluorination using hydrofluoric acid as fluorinating agent.
This type of production method generally requires washing steps in columns provided for this purpose (water or basic aqueous solution) to remove certain undesirable products (residual HF in particular). These washing steps may lead to hydrofluoroolefin compositions comprising water and/or moisture.
As a result, methods for producing hydrofluoroolefin compositions generally comprise one or more drying steps after the washing steps, to obtain streams that are anhydrous or substantially anhydrous before subsequent treatment steps. The conventional methods used for drying are for example the use of a compound having dehydrating properties such as calcium chloride, magnesium sulfate, sodium sulfate, calcium sulfate, potassium hydroxide or calcium oxide. Other well-known methods involve the use of a molecular sieve, silica gel or activated carbon.
In addition, the methods generally used for producing hydrofluoroolefin compositions lead to the formation of by-products (impurities) of hydrocarbon compound type, requiring subsequent purification steps.
Document JP2013241390 describes a method for purifying a stream comprising 2,3,3,3-tetrafluoro-1-propene (1234yf) obtained by pyrolysis. Document WO 2013/115048 describes a method for purifying a stream of 1-chloro-3,3,3-trifluoropropene (1233zd) using a zeolite. In this document, separate steps of washing and drying are applied before it is possible to carry out the purification step.
There is therefore a need to simplify the method for producing hydrofluoroolefin compositions, for example by limiting the number of steps whilst maintaining an efficient method that affords satisfactory yield, in particular for industrial applications.
The present invention sets out to propose a novel method for purifying and drying hydrofluoroolefin compositions allowing both drying to be obtained and the removal of impurities of hydrocarbon type, in particular saturated halogenated hydrocarbons. Therefore, in the method of the invention, purification and drying are conducted simultaneously in one same step.